1. Field of the Invention
The present invention relates to an optical element and a lens which diffracts and converges laser beams with different wavelengths and an optical head device which is used for recording on or reproducing from an optical recording medium such as a CD or a DVD having substrate thicknesses different from each other by means of using laser beams with different wavelengths.
2. Description of Related Art
CDs and DVDs having transparent protective layers with different thickness for protecting their recording surfaces and having different recording densities, are known as optical recording media. A laser beam with a wavelength of 655 nm is used to reproduce data from a DVD and a laser beam with a wavelength of 785 nm is used to reproduce and record data on a CD (CD-R is included).
For example, a conventional optical head device for performing recording and reproduction of data on or from an optical recording medium is constructed so that a laser beam converges on the recording surface of a CD or a DVD by means of a common objective lens to attain its miniaturization and compactness.
The thickness of the transparent protective layer of a CD for protecting its recording surface is 1.2 mm. The thickness of the transparent protective layer of a DVD is 0.6 mm, which is thinner than that of the CD, and its recording density is higher than that of a CD. Accordingly, an objective lens has been proposed in which a diffraction grating including concentrically circular fine step portions are formed on an incident side refraction face provided with a prescribed shape of an aspheric surface. With the objective lens, the incident light beam is diffracted by the diffraction grating to satisfactorily focus on the recording surface of two types of the optical recording media having a thickness of the transparent protective layer different from each other.
In order to construct such an objective lens, the shape of the aspheric surface expressing the refraction shown in FIG. 5(a) is obtained at the time of the optical design by using the following expression:
            z      ⁡              (        h        )              ⁢                  h        2                    r        (                  1          +                                    1              -                                                1                  +                                                            (                      k                      )                                        ⁢                                          h                      2                                                                                        r                  2                                                                    )              +      (                  ∑                  i          =          0                1            ⁢                        a          i                ⁢                  h                      (                          2              ⁢              i                        )                                )  Further, the ray tracing is performed with the optical path difference function representing the diffraction, which is expressed as the following expression, to make a correction such that the aberration may be zero.
      Θ    ⁡          (      h      )        =      λ    ⁢                  ∑                  i          =          0                1            ⁢                        b          j                ⁢                  h                      (                          2              ⁢              j                        )                              Then, the result is replaced in the shape of the blaze working as shown in FIG. 5(b). The shape correction corresponding to the incidence angle is performed on the refraction face in the shape of the blaze shown in FIG. 5(b). Here, “r” means the radius of curvature of the center, “k” means the conical coefficient, “h” means the height from the optical axis, “λ” means the wavelength, and “ai” and “bj” mean the coefficient of the polynomial.
When the blaze height is determined with the wavelength λB, the height of the blaze component from the aspheric surface is calculated by the following expression:φ(λB)/(n−1)wherein “φ” is the optical path difference function and “n” is the refractive index. In this case, the “λB” is referred to as the blaze wavelength.
However, according to the conventional optical design, the blaze wavelength is determined by the following relationship:λ1<λB<λ2;
wherein λ1=the wavelength of the laser beam for a DVD λ2=the wavelength of the laser beam for a CD                Therefore, when the diffraction face is used with the wavelength λ1, the difference between the case at is blazed with the wavelength λ1 and the case that is blazed with the wavelength λB causes a large spherical aberration SA1 for the wavelength λ1. In addition, when the diffraction face is used with the wavelength λ2, the difference between the case that is blazed with the wavelength λ2 and the case that is blazed in the wavelength λB causes a large spherical aberration SA2 for the wavelength λ2. As a result, at the time of recording and reproduction for a DVD, desired pickup characteristics are not obtained due to the large aberration.        